The method and apparatus as embodied in the present invention relates to carburizing by absorption and thermal diffusion of carbon into a metal article in a subatmospheric environment, wherein a carburized case on the metal article is inexpensively and effectively produced.
Changing the surface chemistry of a metal article by thermal diffusion of carbon into the surface of the article so that a hardened case may be produced has long been known in the art, and it is inherent in the case hardening of steel, for example, by carburizing, that the carburizing medium supply an adequate quantity of carbon for adsorption and diffusion into the steel. Prior to the instant invention, carburizing processes have included liquid carburizing using a salt bath, gas carburizing using a gaseous medium as a source of carbon, and pack carburizing using a powdered material as a source of carbon.
The present invention has particular application in gas carburizing and is an improvement over the prior known processes that subjected the metal articles to be treated to a carburizing atmosphere that was force circulated by a fan in the chamber in which the articles were located. Usually a hydrocarbon gas as found in natural gas was employed in the prior known carburizing processes, and in most gas carburizing furnaces a carrier gas such as an endothermic gas was used in combination with the hydrocarbon gas as the carburizing medium, and this medium was circulated in the furnace chamber and under prescribed time and temperature conditions.
It is known that the rate of absorption and diffusion of carbon into steel is an exponential function with respect to the carburizing temperatures. Hence, the higher the temperatures, the greater the rate of absorption and diffusion. However, present furnaces for use in carburizing are limited in the operating temperature ranges, and prior to the instant invention such furnaces have been limited to operating in the range of 1750.degree. F.-1850.degree. F. Any increase in the operating temperature range of the prior known furnaces would cause the refractory brick heating elements and parts in the furnace heating chamber to break down. Although conventional carburizing furnaces could be constructed to withstand high temperatures, the cost thereof would be prohibitive.
The prior known atmosphere furnaces which used an endothermic atmosphere as the carrier gas required long purge cycles to rid the furnace of foreign gases and to establish an acceptable environment for carburizing. This necessarily increased the time factor in carburizing which has been one of the main objections to the prior known carburizing processes.
The prior known carburizing furnaces which were gas or oil fired used multi-zone arrangements which considerably increased the size of the furnace over a conventional heat treating furnace. Further, the endothermic atmosphere used continually polluted the air as it was vented through stacks and required some kind of pollution control which again increased the cost of the process.